1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device, and more specifically, to an isolation film in a semiconductor device and method of forming the same.
2. Discussion of Related Art
Generally, as the size of a NAND flash memory device shrinks, it is required that a depth of an isolation film in a cell region and a depth of an isolation film in a peripheral region be different in the process of forming the isolation film.
An isolation film has a dual depth at a boundary portion of the cell region and the peripheral region. If an isolation film having a dual depth is formed at a well boundary portion in a NAND flash memory, the following problems occur.
First, lots of well leakage is generated due to dislocation of a semiconductor substrate. Holes are formed in the well structure due to damage generated in an etch process for forming an isolation film having a dual depth or damage generated because of shortage of thermal budget. Accordingly, if a voltage is applied to the structure, a leakage path can be formed, which may degrade the voltage.
In view of the characteristics of a NAND flash memory, word lines within a cell array are connected up to a X-decoder. If a high bias voltage is applied to the word lines, a parasitic transistor can be formed at a boundary portion of an isolation film having a dual depth. This can degrade the performance of the operation of the memory.
Accordingly, there is a need for technologies capable of solving the problems generated since an isolation film having a dual depth is formed at a boundary portion of a cell region and a peripheral region.